Process for producing an ultramicrocellular structure by extruding a crystalline polymer solution containing an inflatant

ABSTRACT

AN ULTRAMICROCELLULAR STRUCTURE, THE CELLS OF WHICH CONTAIN AN INFLATANT WHOSE PERMEABILITY COEFFICIENT FOR DIFFUSION THROUGH THE CELL WALLS IS LESS THAN THAT OF AIR, CAN BE OBTAINED BY FLASH-EXTRUDING A SOLUTION OF A POLYMER IN AN ACTIVATING LIQUID, WHICH SOLUTION ALSO CONTAINS THE INFLATANT. UPON EXTRUSION, THE CELLS OF THE STRUCTURE ARE PARTIALLY COLLAPSED BUT UNRUPTURED. UPON EXPOSURE TO AIR THE STRUCTURE SELF-INFLATES DUE TO DIFFUSION OF AIR INTO THE CELLS.

United States Patent Olfice U.S. Cl. 264-45 9 Claims ABSTRACT OF THEDISCLOSURE An ultramicrocellular structure, the cells of which containan inflatant whose permeability coeflicient for diffusion through thecell walls is less than that of air, can be obtained by flash-extrudinga solution of a polymer in an activating liquid, which solution alsocontains the inflatant. Upon extrusion, the cells of the structure arepartially collapsed but unruptured. Upon exposure to air the structureself-inflates due to diffusion of air into the cells.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a divisionof my application Ser. No. 615,883, filed Feb. 14, 1967, now Pat. No.3,375,211, which is in turn a continuation-in-part of application Ser.No. 302,720, filed Aug. 16, 1963, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a novel processfor producing a class of ultramicrocellular structures containing aparticular volatile constituent within the cells. It also relates to aprocess for producing such structures which are provided in a collapsedcondition so as to be capable of selfinflation.

Crystalline polymeric ultramicrocellular structure and methods for theirproduction are described in US. Pat. 3,227,664, issued J an. 4, 1966.The ultramicrocellular structures are particularly unique owing to thepolyhedral shaped structure of their cells, to the film-like characterof the cell walls, and to the uniform texture and high degree ofmolecular orientation, i.e. uniplanar orientation, existing in thoseWalls. Together these features serve to define a class of materialswhich, in comparison with prior art cellular structures, exhibitoutstanding strength and resiliency properties although fabricated atextremely low densities. From the standpoint of still other desirablecharacteristics, the ultramicrocellular structures are supple, opaque,pneumatic, and have an exceedingly high bulk and low thermalconductivity (or k factor). Because of this superior combination ofproperties and the fact that they can be produced in sheet, filament orother shaped or bulk form, the ultramicrocellular structures are welladapted to a great variety of end uses.

SUMMARY OF THE INVENTION An object of the invention is to obtain animproved process for producing a class of ultramicrocellular structuresby the provision of a particular 'volatile constituent within the cells.A further object of the invention involves the production ofultramicrocellular structures in a form such that substantial reductionsin density can be realized. A further object of the invention relates tothe production of such structures which are provided in a substantiallycollapsed, relatively dense form such that they can be 3,584,090Patented June 8, 1971 self-inflated to a low density pneumatic form atthe point of use. It is a further object to provide a process forpreparing and handling such self-inflatable structures. It is also anobject to produce storage stable articles wherein the ultramicrocellularcomponent can be self-inflated either spontaneously in air or by merelyheating in air. It is a still further object to produce collapsedultramicrocellular structures which can be self-inflated to apredetermined shape and size, or which can be self-inflated within aconfining volume such that the structure expands to take on the shape ofthat volume. Other objects will be apparent from the remainder of thespecification and claims.

In accordance with the invention, there is provided anultramicrocellular structure composed of a high molecular weightsynthetic crystalline polymer and having a major proportion of closedpolyhedral cells defined by air permeable walls having a thickness ofless than 2 microns with essentially all the polymer constituting cellwalls and exhibiting uniform texture and uniplanar orientation, saidcells containing an inflatant whose permeability coeflicient fordiffusion through said walls is less than that of air, said inflatantbeing capable of generating a vapor pressure of at least 30 mm. Hg at atemperture below the softening point of said polymer and being selectedfrom the group consisting of sulfur hexafluoride and saturated aliphaticand cycloaliphatic compounds having at least one fluorine to carboncovalent bond and wherein the number of fluorine atoms exceeds thenumber of carbon atoms.

The foregoing structures containing the described inflatants may varyfrom extremely low density/high bulk materials of only 0.02 g./ cc. orless, e.g. as obtained in the case where self-inflation is allowed tocontinue unimpeded following cell formation, to relatively high densitymaterials, e.g. as obtained in the case Where self-inflation of acollapsed cellular structure is restrained by preventing the osmoticdiffusion of air into the cells after their formation. The latter classof high density materials is further characterized by the presence ofpartially collapsed but unruptured polyhedral cells and by an ability tobe postinflated to less than one-half its collapsed density withoutsubstantial stretching of the cell 'walls by having an Nc-s productgreater than about 30, wherein Nc is the number of cells per cc. in thecollapsed structure and s is the average cell surface area in squarecentimeters.

Also in accordance with this invention there is pro vided a process forobtaining the above-mentioned collapsed, self-inflatableultramicrocellular structure. The process is an improvement on the flashextrusion process described in aforementioned US. Pat. 3,227,664 whereina solution of a polymer in an activating liquid, said solution beingmaintained at super-atmospheric pressure and at a temperature above theboiling point of said activating liquid, is discharged from an orificeinto a region of lower pressure and temperature to precipitate saidpolymer. The improvement comprises providing in said solution aninflatant of the type hereinabove described. Immediately followingextrusion, the structure collapses owing to condensation of theactivating liquid and/or diffusion thereof through the cell walls. Theinflatant, however, remains within the cells and provides an osmoticdriving force for diffusion of air into the cells, and consequentinflation of the structure.

DETAILED DESCRIPTION Among the numerous advantages afforded by theinvention, it is first of all significant that the utilization ofparticular volatile inflatants within the cells serves to permit lowerdensity values than would otherwise be achieved. Aside from the obviouseconomic benefits which are thereby realized in those applications inwhich bulk is of importance, improved functional properties,particularly with respect to insulation uses, are also achieved. In thecase where the infiatant is selected to be of an impermeant character,i.e. so that there is little or no tendency for it to diffuse out of thecells, then a further advantage is obtained in that the products can besuccessively compressed and released under a load without a permanentloss in pneumaticity. Still a further and highly unexpected benefitarising from the use of the inflatants is that a simple and effectivemeans is thus obtained for overcoming the costly and cumbersome problemsheretofore associated with the handling, storage and transportation, ofultramicrocellular structures. This benefit is achieved by initiallyobtaining the structures in a storage stable, self-inflatable, collapsedform rather than in the high-bulk form otherwise produced. When andwhere desired, the ultimate users of the product can then effectspontaneous inflation thereof to the desired degree, e.g. by simplyopening an air impervious package containing the collapsed material. Forsome purposes, the collapsed self-inflatable structures possess stillanother important advantage; namely, that the structures can be expandedwithin a confining .volume to fully occupy the space therein.

The ultramicrocellular structures of the invention are desirablyproduced by the extrusion of a solution containing a high molecularweight synthetic crystalline polymer, an infiatant as hereinbeforedescribed, and an activating liquid for which the polymer is highlypermeable. The solution, maintained at a temperature above the boilingpoint of the activating liquid and at a pressure substantially aboveatmospheric pressure, is extruded through an orifice into a region oflower pressure and temperature, normally room temperature at about oneatmosphere. Immediately upon extrusion into the low pressure andtemperature region, substantially all of the activating liquidevaporates adiabatically. As a result, a large number of tiny vaporbubbles, usually at least about 10 bubble nuclei per cc., are created inthe solution and sufficient heat is absorbed to lower the temperature ofthe confining polymer below the polymer melting point. This step occursvery rapidly, usually in less than seconds, and therefore effectivelyfreezes-in the molecular orientation generated in the polymer cell wallsduring the bubble expansion. With a short period of time, usually lessthan about 10 seconds or so, the structure will tend to partiallycollapse to a transient or unstable condition owing to condensation ofthe activating liquid and/ or partial diffusion thereof through the cellwalls. A substantial proportion or all of the slower diffusing infiatantremains as a confined residue within the cells. If a large amount ofgaseous infiatant remains in the cells, then this will override thetendency of the cells to collapse and actual collapse will not occur. Ineither case continued exposure of the extrudate to the air atmospherewill give rise to a fully inflated product since the infiatant vaporscreate an osmotic driving force which facilitates entry of air moleculesinto the cells. When a fully inflated sample is so obtained directlyfollowing the extrusion operation, the product will exhibit a lowerdensity than a corresponding structure prepared from a solutioncontaining no such infiatant. Even in the fully expanded form the roleof the infiatant is not complete so long as vapors thereof persist inthe cells; that is, the structure will self-inflate in air even thoughit has been maintained under a compressive load to such an extent that aconsiderable quantity of air has escaped from the cells.

In a preferred embodiment of the invention, the choice and quantity ofinfiatant are such that upon condensation and/or diffusion of theactivating liquid immediately following extrusion, the structure willactually collapse even though exposed to air. During the period when thedensity of the resultant collapsed structure is at least twice thatdesired in the product upon post-inflation, preferably while the densi yis at a maximum, Self-inflation is re- 4 strained by preventing osmoticdiffusion of air into the cells. Thus in one embodiment the transcientcollapsed condition can be preserved by enclosing the material in a gasimpervious barrier, e.g. a plastic bag or other sealed container fromwhich air is excluded. Depending upon the exact relative permeationrates of the activating liquid and air as well as the original quantityof each in the cells, the stage of maximum or near maximum collapse canpersist for an appreciable period of time -(e.g. a few minutes to anhour or so), and the imposed restraint may successfully be effected atany time within this period.

General process parameters for mixing and extrusion techniques aredescribed in aforementioned US. Pat. 3,227,664.

The manner by which the ultramicrocellular structures can self-inflateto a maximum in air, either during extrusion or at a later time, dependsnot only upon the particular configuration and construction of the cellsbut also upon the interaction of the confined infiatant and thesynthetic polymer which forms the cell walls. Still an additional factorwhich contributes to the self-inflatable characteristic involves theper-meant nature of air as a post-inflation atmosphere; moreparticularly, the ability of air to diffuse into the cells of thestructure before the infiatant can diffuse outwardly therefrom. In thisrespect it is to be understood that the so-called air permeabilityproperty of the cell walls of the structures of the invention isattributed not to porosity in the sense of openings, but rather to theability of the walls to selectively permit diffusion therethrough ofcertain vapors. In effect the walls are semi-permeable such that airwill osmotically diffuse from a high air partial pressure side of a wallthrough the wall to a low air partial pressure side of that wall untilthe respective fugacities are equal. On the other hand, osmoticdiffusion through the walls by a highly impermeant gas will berestrained irrespective of whether or not such a partial pressuredifferential exists.

With regard first to the unique ultramicrocellular structure of theproducts of the invention, substantially all of the polymer is presentas filmy elements whose thickness is less than 2 microns, preferablyunder 0.5 micron. The term drained foam is aptly descriptive of suchultramicrocellular structures. The thickness of a cell wall, bounded byintersections with other walls, does not ordinarily 'vary by more thani30%. Adjacent walls frequently will have generally equal thicknessvalues, such as within a factor of 3. The polymer in the cell wallsexhibits uniform texture and uniplanar orientation. The apparent densityof the ultramicrocellular products is between 0.5 and 0.005 g./cc. Thenumber of cells per cc. in expanded condition, Ni, is desirably at least10 although values of 10 or greater are preferred.

The cell wall thickness can be determined by microscopic examination ofcross sections. With a deflated sample such measurements are facilitatedby first inflating the sample as later described. Thus 2060-micron thicksections may be cut from a frozen sample with a razor blade. Large cell50 microns) samples are frozen directly in liquid nitrogen. Smallercelled samples are preferably imbedded in water containing a detergent,and then frozen and sectioned. The transverse dimension of one or morecells can also be readily measured by freezing and sectioningtechniques. The cells are found to exhibit a general polyhedral shape,similar to the shape of the internal bubbles in a foam of soap suds. Inorder for inflated ultramicrocellular products to be supple, opaque andstrong, the cell dimensions must be small compared to the smallestexternal dimension of the product. For this reason the averagetransverse dimension of the cells in expanded condition should be lessthan 1000 microns, preferably less than 300 microns, and the mutuallyperpendicular transverse dimensions of a single cell in a fully inflatedcondition should not vary by more than a factor of three. The ratio ofthe inflated cell volume of the cube of the wall thickness can becalculated a d e ceeds about 200. For very thin walled samples 1micron), the wall thickness is preferably measured with aninterferometer microscope. A layer of the sample is peeled off bycontact with Scotch Tape. The layer is freed from the tape by immersionin chloroform and subsequently placed on the stage of the microscope formeasurement.

The term uniplanar orientation employed with respect to the products ofthis invention may be fully under stood from the following discussion.Axial, planar, and uniplanar indicate different types of molecularorientation of high polymeric materials. Axial orientation refers to theperfection with which the molecular chains in a sample are aligned withrespect to a given direction, or axis, in the sample. For example, priorart filaments which have been drawn in one direction only generallyexhibit an appreciable degree of axial orientation along the stretchdirection. Planar orientation refers to the perfection 'with 'which themolecular chains are oriented parallel to a surface of the sample.Uniplanar orientation as possessed by the products of this invention, isa higher type of polymer orientation in that it refers to the perfectionwith which some specific crystalline plane (which must include themolecular chain) in each polymer crystallite is aligned parallel to thesurface of the sample. Obviously, only crystalline polymers can exhibituniplanar orientation. These three types of molecular orientation mayoccur singly or in combination; for example, a sample mightsimultaneously exhibit uniplanar and axial orientation.

Electron diffraction furnishes a convenient technique for observing thepresence of uniplanar orientation in the microcellular structures ofthis invention. A single cell 'wall is placed perpendicular to theelectron beam. Since the Bragg angle for electron diffraction is sosmall, only crystalline planes essentially parallel to the beam(perpendicular to the wall surface) will exhibit diffraction. If thesample does in fact have perfect uniplanar orientation, there is somecrystallographic plane which occurs only parallel to the film surfaceand, therefore, will be unable to contribute to the diffraction pattern.Thus, the observed pattern will lack at least one of the equatorialdiffractions normally observed for an axially oriented sample of thesame polymer. If the degree of uniplanar orientation is somewhat lessthan perfect, there may be a few crystallites tilted far enough tocontribute some intensity to the diffraction pattern, but at least oneof the equatorial diffraction intensities will be appreciably less thannormal. Thus, for the purpose of this invention, a sample is consideredto have uniplanar orientation when at least one of the equatorialdiffractions appears 'with less than one-half its normal relativeintensity as determined on a standard -which is a randomly orientedsample of the same polymer. A simple standard for this purpose is athick inflated portion of the same ultramicrocellular structure, sincethe total array of many walls averages out to random orientation;necessarily the intensities of the diffraction pattern for such astandard will be made using an X-ray beam because of the increasedsample thickness.

An alternative and occasionally more convenient technique for detectingthe presence of uniplanar orientation in a sample is to observe theelectron diffraction pattern as the plane of the sample is tilted withrespect to the electron beam. (In case the sample also exhibits axialorientation, the tilt axis is preferably parallel to the orientationaxis.) For uniplanar-oriented samples, first one crystallographicdiffraction plane and then another will assume the position required forBragg diffraction, so that first one and then another lateraldiffraction will appear and then disappear as the sample rotationcontinues. The more perfect the degree of uniplanar orientation, themore sharply defined is the angle at which any particular diffractionappears. When a plot of diffraction intensity (corrected for samplethickness variation) vs. angle of sample tilt is prepared for any of thelateral diffractions, the distance in degrees tilt between points ofhalf-maximum intensity may be readily determined. Only samples havinguniplanar orientation will have halfmaximum intensity points separatedby or less, and this will serve as an alternate criterion for thepresence of uniplanar orientation.

One precaution must be observed in making this measurement. If thesample field examined by the electron beam is stopped down so far thatit sees only one crystallite at a time, it will always be possible, evenfor a randomly oriented sample, to find some crystallite orientedparallel to the sample surface which would, of course, give an uniplanarorientation diffraction pattern. In order to insure that the uniplanarorientation pertains to the whole film element and not just to onecrystallite, the measurement should be made examining a field of atleast square microns area, which is large enough to include thecontributions from many crystallites simultaneously. Other techniques ofmeasuring uniplanar orientation and their co-relation with electrondiffraction measurements are described in the J. Pol. Sci. 31, 335(1958) in an article by R. S. Stein.

The term uniform texture applied to the polymer in the cell walls meansthat the orientation, density, and thickness of the polymer issubstantially uniform over the whole area of a cell wall, examined witha resolution of approximately /2 micron. This is best determined byobserving the optical birefringence in the plane of a wall of a cellremoved from the sample. For ultramicrocellular samples with a netover-all axial orientation, the individual cell walls will also normallyexhibit an axial orientation in addition to the required uniplanarorientation. In the birefringence test, such products of the presentinvention will show a uniform extinction over the whole area of the cellwall. Samples with no net axial orientation must show a uniform lack ofbirefringence over their whole area rather than numerous small patchesof orientation with each patch oriented at random with respect to theothers. Lacy or cobweb-like cell walls, of course, do not have uniformbirefringence over the whole area of a cell wall, and such products arereadily distinguished from the uniform textured products of thisinvention.

It is characteristic of and essential to the products of this inventionthat the cells be of the closed variety and be unruptured even in acollapsed condition. By closed is meant that at least a major proportionby number of cells in any ultramicrocellular sample possess a pluralityof defining walls, i.e. unruptured walls, which wholly encapsulate aninner space or void. For practical purposes mere visual or microscopicexamination will often readily reveal "whether or not a particularcellular structure predominates in closed or open cells. Particularlythis is true in the case when the identity of the polymer and theconditions of cell formation are known. Otherwise the closed-cellcontent of a yieldable sample may be determined by the gas displacementmethod of Remington and Pariser, Rubber World, May 1958, p. 261,modified by operating at as low a pressure differential as possible tominimize volume changes of the yieldable closed cells.

Those structures which are in a collapsed condition are recognizable assuch because of their ability to be postinflated when osmotic diffusionof air thereinto is no longer restrained. Even repeated steps ofinflation and deflation performed upon a given sample fail to destroy orrupture a significant number of cell walls.

The collapsed untramicrocellular structures of the invention areself-inflatable to less than one-half their collapsed density by havingan Nc-s product greater than 30, wherein N0 and s are as hereinbeforedefined. Fulfillment of this limitation in effect ensures that the cellsare in a sufficiently collapsed condition to be postexpandable to atleast twice their collapsed volume without substantial stretching of thecell walls.

It should be noted that the expression Nc-s 30, is essentiallyindependent of cell geometry so long as the cells are polyhedral andhave film-like walls of the maximum thickness values specified herein.In practice the values of No and s need not be determined directly forcellular samples but rather can be calculated from other known ordeterminable factors, e.g.:

sis the average cell surface area in cm.

is the average density of the foam sample at any convenient degree ofinflation m,

po is the bulk polymer density,

2 is the average cell wall thickness in cm.,

Nm is the number of cells per cc. in the sample at the same degree ofinflation m,

N is the number of cells per cc. in the sample in a collapsed condition,

Ni is the number of cells per cc. in the sample in inflated condition,

p is the density of the sample in collapsed condition,

and

p is the density of the sample in inflated condition.

When these definitions are inserted in the previous inequality relationcharacterizing collapsed cellular structures, simple algebraicmanipulation yields:

p 2 pm pom 112 This provides an entirely equivalent definition ofcollapsed cellular structures in which only directly observableparameters appear, and which therefore is ordinarily more convenient toemploy.

Values of p for a sample of an ultramicrocellular structure at anydegree of inflation are conveniently obtained by measuring the volume ofwater that a given weight of sample displaces. Values of Nm can bedetermined by microscopic examination. The latter measurement willordinarily be easiest to perform when the sample is in a fully inflatedcondition. Alternatively, a sufficiently accurate approximation for aninflated sample is Nm=NiEl/d where d is the average cell diameter. Thevalue obtained can then be used to calculate Ne from the equation Nc=Nm/p From simple geometrical arguments, it can be shown that the quantityNi 1 is quite insensitive to cell geometry, and ranges only from 13.7 to15.8 for fully inflated closed cell structures with assumed shapesranging from square to hexagonal cross sections and ratios of length towidth up to 2:1. Even a grossly disproportionate cell elongation ratioof 5:1 would only provide a Ni -s quantity of 20.7. Therefore therestriction that Nc-s requires that the cell walls have been crumpledand the structures collapsed so that the number of cells per cc. is atleast twice that of the fully inflated structures, i.e. the product canbe reinflated to at least twice its collapsed volume without generatingany new cells or appreciably stretching any of the pre-formed crumpledwalls existing in the partially collapsed structures. The preferredcollapsed products will have Nc-s values greater than 60, i.e. can bepost-reinflated to at least 4 times their collapsed volume.

The polymers employed in accordance with the invention are members ofthe class of high molecular weight synthetic crystalline polymers. Sincethe polymer walls of the ultramicrocellular structures are film-likewith a thickness of less than 2 microns, the polymer must accordingly beof at least film-forming molecular weight.

A further requirement of the polymer relates to the strength andresiliency which cell walls thereof impart to the ultramicrocellularstructure. Thus the polymer must impart sufficient strength to thestructure that the particular cellular nature thereof not be destroyedupon inflation, e.g. so as to resist rupture or a significant stretchingof the cell walls. For this reason the polymer should have a yieldstrength of at least 1000 p.s.i. as measured by the test method of ASTMD638-58. On the other hand, for inflation to occur such that maximumbulk values are obtained, the polymer composed structure must be suchthat in expanded condition it be yieldable, e.g. resilient such thatsubstantial deformation occurs under internal-external pressuredifferentials, meaning differences, of one atmosphere or less (sincethis is the order of magnitude of the pressure differentials availablefor collapse and inflation). By substantial deformation is meant thatthe ultramicrocellular structure in expanded condition, i.e. having aninternal pressure of at least about one atmosphere with few if anybuckles and wrinkles in the cell walls, is yieldable such that itsvolume can be compressed by at least 10% under a load of 10 pounds persquare inch sustained for a period of 1 second with recovery of at leastabout 50% of its original volume on release of the load. Structureswhich do not compress to that extent are entirely too rigid and hence donot afford a suflicient degree of resiliency to respond to pressuredifferentials. Moreover, if it does not sufliciently recover afterrelease of the load, then it is not sufliciently flexible to resistfracturing and rupturing of the cell walls.

An essential feature of the polymer which constitutes the cell walls isthat it exhibits selective permeability to different gases; inparticular, be permeable with respect to air but less permeable withrespect to inflatant vapors. Without this feature efforts to achievefull expansion would be unsuccessful for the reason the inflatant wouldbe prematurely lost before sufiicient air had entered the cells. Theclass of crystalline and crystallizable polymers is well suited forachieving this function.

Examples of synthetic organic polymers suitable for producingultramicrocellular structures in accordance with this invention thusinclude the class of synthetic crystallizable, organic polymers; e.g.polyhydrocarbons such as linear polyethylene, stereo-regularpolypropylene or polystyrene; polyethers such as polyformaldehyde; vinylpolymers such as polyvinylidene fluoride; polyamides both aliphatic andaromatic, such as polyhexamethylene adipamide and the polyamide from2,2, bis p-aminophenyl propane and isophthalic acid; polyurethanes, bothaliphatic and aromatic, such as the polymer from ethylenebischloroformate and ethylene diamine; polyesters such aspolyhydroxypivalic acid and polyethylene terephthalate; copolymers suchas polyethylene terephthalate-isophthalate, and equivalents. Thepolymers should have a softening point of at least about 40 C., asindicated by passing a stick of solid polymer in sliding contact with aheated metal bar and observing the bar temperature at which a streak ofmolten polymer is first formed. Polymer properties such as solubility,melting point, etc. are usually reflected in the properties of theultramicrocellular product.

One of the features of the ultramicrocellular structures is the highdegree of orientation of the polymer in the cell walls, whichcontributes to the unique strength of these structures. Therefore, apreferred class of polymers includes those materials which respond to anorienting operation (e.g., drawing of fibers or films) by becomingsubstantially tougher and stronger. This class of polymers is well knownto one skilled in the art and includes, for example, linearpolyethylene, polypropylene, 66 nylon, and polyethylene terephthalate.

As is clear from the foregoing disclosure, the polymers suitable for usein preparing the cellular structures of this invention must have areasonable permeability to air at 9 room temperature, e.g. have adiffusion coefiicient for nitrogen of at least 10- coi /cm. sec./cm./cm. Hg

The function of the inflatant contained with in the cells of theproducts of the invention is to afford an osmotic driving force which iscapable of causing inflation of the structure, e.g. during the extrusionoperation or at a later time. Accordingly such inflatant must berelatively impermeant by which is meant that at C. its permeabilitycoefficient for diffusion through the cell walls be lower than that ofair. Thus the vapors of the inflatant must be incapable of permeatingthe cell walls, e.g. outwardly from the sample, as fast as air canpermeate the cell walls, e.g. into the sample, for otherwise fullinflation of the cells could not be achieved upon continued exposure ofthe sample to an air atmosphere. An impermeant inflatant is one whosepermeability coeflicient for diffusion through the cell walls is notonly lower than that of air but also is incapable of permeating the samecell walls at room temperature, e.g. below 40 C., at such a rate that /2or more thereof will escape to an air atmosphere by diffusion Within 1days time, preferably 1 months time or longer. The latter guarantees ineffect that the structure, whether collapsed or not, will retain itsinflation activator (the impermeant inflatant) for a reasonable storageperiod. Considerable economic benefits accrue from being able to workwith collapsed post inflatable filaments or sheets in a primaryproduction area, particularly with regard to rewinding and web slittingoperations. For such purposes an impermeant inflatant lifetime of 1 dayis ample. However for inventory, storage purposes and shippingoperations, longer lifetimes are usually required, e.g. of one month orlonger.

If the inflatant has essentially zero permeability through the cellwalls at room temperature and atmospheric pressure, as is preferred, afurther advantage is gained; namely, the sample can be successivelyinflated and deflated as desired either by mere application and releaseof a load or, alternatively, by the respective steps of simply exposingto air and removing from contact with air. Necessarily the inflatantmust not be a solvent for the polymer under such conditions that thesample will be exposed to following discharge from the extrusionorifice.

The minimum quantity of inflatant contained in each cell is ofsignificance in realizing an adequate osmotic driving force to obtainreliable and reasonably rapid selfinflation to the fullest extent. Thusa major number of the cells should contain at least some quantity of theinflatant. Since the inflatant must exist in a gaseous condition tocreate an osmotic driving force, it must be either a gas at roomtemperature or be capable of vaporizing at a temperature below themelting point of the synthetic polymer defining the walls. In practiceit has been found that the inflatant must be capable of generating avapor pressure of at least mm. Hg at a temperature below the softeningpoint of the polymer in order to reliably provide well defined cavitiesinto which the external air will diffuse in reasonable periods of time.Therefore, if the vapor pressure of the inflatant in the partiallycollapsed cells is at least 30 mm. at room temperature, as is preferred,the structure will normally self-inflate in air as desired. However,inflatants whose vapor pressures are less than 30 mm. at roomtemperature may also be successfully employed although frequently anadded step of briefly heating the structure, above room temperature butbelow the polymer softening :point, to increase the partial pressure ofthe inflatant will assist in reducing the time necessary for fullinflation to occur.

Higher internal inflatant pressures are of course operable, and are infact preferred, particularly when the confining cellular structure haswalls near the upper thick- 11 Ce. of gas at STP.

b Cm. of surface.

c Cm. of sample thickness.

11 Pressure difference across sample.

ness limit (2 microns) or when the polymer comprising the wallspossesses a high flexural modulus (above 100,000 p.s.i.) at roomtemperature. In general, the solution of polymer to be extruded shouldcontain about 1 to 20% by weight of the inflatant.

The rate of permeation for an inflatant through a given polymerincreases as its diffusivity and solubility increase. Accordingly,candidates for inflatants should have as large a molecular size as isconsistent with the required 30 mm. minimum vapor pressure, and havelittle solvent power or affinity for the confining polymer cell walls. Apreferred class of such inflatants is exemplified by compounds whosemolecules have chemical bonds different from those found in theconfining polymer, a low dipole moment, and a very small atomicpolarizability. Furthermore, it is advantageous, though not necessary,that the inflatant be a high vapor pressure solid or liquid underambient conditions in order that small quantities of solid or liquidphase inflatant may be present in each partially collapsed cell, inaddition to the inflatant vapor. For such systems the inflatant vapor isreplenished from the solid or liquid inflatant reservoir as thecollapsed structure inflates, thus maintaining the full osmotic drivingforce up through the stage of full inflation. Materials which arenon-gaseous at room temperature and atmospheric pressure are preferredsince for a given weight they occupy less volume; hence, the bulk ofcollapsed structures can be appropriately minimized. Products containingthe normally liquid or solid inflatants need at most only be heated inthe atmosphere to vaporize a part of the inflatant and to therebyspontaneously cause inflation.

Suitable inflatants according to the invention are selected from thegroup consisting of sulfur hexafluoride and saturated aliphatic andcycloaliphatic compounds having at least one fluorine to carbon covalentbond and wherein the number of fluorine atoms exceeds the number ofcarbon atoms. Preferably the saturated aliphatic and cycloaliphaticcompounds are, respectively, perhaloalkanes and perhalocycloalkanes inwhich at least of the halogens are fluorine. Although the aliphatic andcycloaliphatic inflatants may contain ether-oxygen linkages, they arepreferably free of nitrogen atoms, carbon to carbon double bonds andreactive functional groups. Specific examples of inflatants includesulfur hexafluoride, chlorotrifluoromethane, dichlorodifluoromethane,

l,1,2-trichloro-l,2,2,-trifluoroethane, sym-dichlorotetrafluoroethane,perfluorocyclobutane, perfluoro-1,3-dimethyl cyclobutane, andperfluorodimethylcyclobutane isomeric mixtures. Mixtures of two or moreinflatants can often be used to advantage.

Aside from the foregoing characteristics, it will be understood that theinflatants must be inert, i.e., be thermally stable under extrusionconditions, and chemically and hydrolytically stable under ambientconditions. For certain uses it will be recognized that toxic compoundsshould be avoided.

Since an essential function of the activating liquid is to generate thecells upon adiabatic evaporation thereof, it must fulfill the followingcriteria:

(a) The liquid should have a boiling point of at least 25 C. andpreferably at least C. below the melting point of the polymer used;

(b) The liquid should be substantially unreactive with the polymer andinflatant during mixing and extrusion;

(c) The liquid should be a solvent for the polymer under the conditionsof temperature, concentration and pressure suitable in this invention;

(d) The liquid should form a solution which will undergo rapidvaporization upon extrusion.

Activating liquids which have been found suitable for certain polymersin the process of this invention include methylene chloride,fluorotrichloromethane, 2,2-dimethyl butane, pentane, hexane, andmethanol.

The production of collapsed self-inflatable ultra-microcellularstructures requires a judicious selection of inflatant and activatingliquid to ensure that a transient collapsed structure is obtained. Tothis end it is desirable to select an activating liquid which willquickly permeate the cell walls before being fully replaced by air fromthe atmosphere. For this technique the activating liquid should be onewhose vapors have a permeability at least twice that of air through theselected polymer in its nascent condition upon emerging from theextrusion orifice. Alternatively, the structure :may be extruded into anatmosphere of a relatively impermeant gas to facilitate outwarddiffusion of the activating liquid and prevent inward diffusion of airor other gases. In the event a gaseous or liquid nucleation assistantsuch as carbon dioxide or nitrogen is employed to give increasednucleation, it too must be highly permeant or otherwise incapable ofpreventing collapse of the extrudate. Certain non-fluid nucleationassistants such as the silicates which remain as solid particles in theproduct do not, of course, impair spontaneous deflation.

For purposes of obtaining the ultramicrocellular structures in acollapsed condition, an essential feature leading thereto comprisescatching the cellular structures in transient, collapsed,inflatant-containing condition and immediately preventing diffusion ofair into the cells. The transient collapsed state required for theprocess is preferably produced by exposing to air (or an atmosphere ofan even less permeant gas) a freshly extruded, unstable,

inflated cellular structure whose cells contain, in addition to therequired impermeant inflatant, at least /2 atmosphere partial pressureof an activating liquid which permeates the polymer at a rate at leasttwice as fast as air permeates the polymer. Upon such exposure the rapidpermeating agent escapes from the cells at a rate faster than airpermeates in to take its place, thus decreasing the internal gaspressure to the point where external atmospheric pressure crushes andcollapses the cellular structure. At room temperature this collapsedcondition is transient for the reason that air continues to diffuse intothe par-tly collapsed cells to reinflate them, driven by the osmoticpressure gradient created by the continued presence in the cells of theimpermeant inflatant. 'It is one discovery of this invention that thistransient collapsed state may be trapped and confined indefinitely byany of a variety of means serving to prevent osmotic diffusion of airinto the cells.

A particularly suitable technique for restraining selfinflation of thetransient collapsed ultramicrocellular structures involves theutilization of a gas impervious barrier means to surround the structure.Such means can vary widely depending upon such factors as availability,convenience in handling, and the like. Preferably they are in the natureof flexible containers or receptacles adapted to receive the collapsedstructure shortly after extrusion and to be sealed from contact with theair. Although plastic film materials such as bags, pouches or otherwrapping forms are especially suitable, aluminum or other metal foils,resin impregnated fibrous webs and the like can also be employed in theform of suitable containers. Films of polyethylene terephthalate andpolyolefin films coated with polyvinylidene chloride resins constitute ahighly effective packaging medium. Except in the case Where theparticular contents are to be provided with a tightly fitting skin inwhich there is little or no available space for air or other gases, itis desirable to provide in the container an enveloping atmosphere of agas which is impermeant to the cell walls of the ultramicrocellularstructure. Thus by enclosing such a structure in a container which hasfirst been flushed with an impermeant gas, there is no available air todiffuse into the cells and hence any tendency toward post-expansion isrestrained. The considerations involved in the selection of such animpermeant gas are essentially the same as hereinbefore described withrespect to the inflatants. For most purposes 12 it will be desirable touse the identical gas to function both as an inflatant in producing theultramicrocellular structure and as an external atmosphere in packagingthe structure in a container.

It will be apparent that a flexible packaging material such as a plasticfilm is especially advantageous, since introduction of the impermeantgas is facilitated and since overall packaging efficiency can beoptimized. Two features should be observed in the selection of asuitable packaging container. First of all it should be gas imperviousin the sense that it prevents the entry thereinto of permeant gases suchas air which would diffuse into the cells of the confinedultramicrocellular structure and cause inflation. In this regard thepermeability coefficient of the container with respect to air should beessentially zero. On the other hand, no problem would be created shouldit be readily permeated by any residual activating liquid that maydiffuse from the contents. In the case where the relatively impermeantinflatant employed to produce the ultramicrocellular structure wouldslightly permeate the cell walls on long standing, the barrier means orcontainer should serve a second function: namely, that it retain withinthe article sufficient inflatant to cause self-inflation when thecontainer is opened. Thus even though a portion of the inflatant isinitially lost by diffusion from cells of the ultramicrocellularstructure, continued losses will cease when the partial internalpressure of inflatant in the cells equals the partial pressure thereofoutside of the structure and within the container.

For most purposes the packaging of a collapsed ultramicrocellularstructure, for example following extrusion thereof, will be performed atroom temperature and at mospheric pressures. It will be apparent tothose skilledin-the-art that packaging and storage operations can,however, also be performed under conditions of elevated or reducedtemperatures and/or pressures provided that the nature of the selectedcontainer is appropriate.

Particularly in the case where the inflatant is impermeant as abovedescribed, an alternative technique involving the use of mechanicalrestraining forces may be employed for preventing the self-inflation ofa transient collapsed ultramicrocellular structure. So long as theexternal mechanical restraint or confinement is applied, it serves tocounterbalance the osmotic self-reinflation driving force. When thecollapsed structure is eventually released, osmotic self-reinflation toa stable, fully inflated cellular structure will occur as long as asufficient quantity of inflatant remains in each cell. The mechanicalrestraint can be accomplished in any con venient fashion, as by stuflingthe transient collapsed product into a bag or metal cage, or by commonbaling methods employing confining straps. Also found satisfactory, issimply storing the collapsed cellular structure under dead load.Alternatively and in a preferred embodiment, the structure canconveniently furnish its own restraint, as when multiple layers of acollapsed sheet or fiber are wound on a core under tension. In any case,so long as each of the collapsed cells retains its quantity of theinflatant, self-reinflation occurs only when the mechanical restraint isremoved.

A third means of effectively preventing a collapsed ultramicrocellularstructure from self-inflating involves the use of certain crystallinesynthetic polymers to impart sufficient rigidity to the cell walls. Thuswhen the polymer employed has a glass transition temperature of at least40 C., as in the case with polyethylene terephthalate, and a structureis spun which spontaneously collapses, then a metastable product can beobtained which has no tendency to expand in the atmosphere as long asthe material is maintained below its Tg". In practice the metastablecondition can be realized by confining the freshly spun collapsedstructure under a load for a period of time until the activating liquidhas fully diffused therefrom and until the polymer in the cell walls hasbecome set. In such a structure, the osmotic self-reinflation drivingforce is counter-balanced by the mechanical rigidity of the crumpledcell walls as long as the material is maintained below its Tg. Theorigin of the mechanical restraint resides in the high modulus of thecrumpled walls with perhaps some contribution from light surface-bondingbetween internal crumpled cell faces. In any event, the osmoticinflation force prevails over the mechanical restraint when the glasstransition temperature of the sample is exceeded, and the samplereinflates to become stable in fully inflated form.

As regards the provision of collapsed self-inflatable materials, theinvention makes possible the economical production of ultramicrocellularstructures at one central location while still taking advantage ofstoring and shipping the products in a substantially lower bulk form.Other advantages which also accrue from this technique include, forexample, the fact that the collapsed structures will exhibit a volumeexpansion of at least 2 times upon, at most, mere exposure to air at anelevated temperature. Therefore, the collapsed structures can be placedinside a confining shape such as an airplane wing, refrigerator door,life jacket cover, etc., before exposing the sample to air, whereuponthe expansion will completely fill (and reinforce, if adhesives orthermal bonding is employed) the confining structures. A furtheradvantage lies in the fact that the structures can be pre-shaped, forexample by the extrusion process itself, before they are collapsed sothat the identical shape is regenerated on subsequent self-expansion.The ultramicrocellular structures can be extruded in the form of sheetmaterials, filamentary materials, rods, tubes, etc.

Among the numerous modifications which are possible, it will be apparentthat the products of the invention can be provided to contain commonpolymer additives such as dyes, pigments, antioxidants, delusterants,antistatic agents, reinforcing particles, adhesion promoters, removableparticles, ion exchange materials, U.V. stabilizers and the like byinclusion of such with the polymer solution prior to extrusion or byother suitable treatments.

The following examples serve to further illustrate this invention.Examples I to VIII demonstrate the preferred embodiment wherein theinflatant is provided in the cells directly by the extrusion procedure.An alternative technique involving introduction of inflatant into thecells of an already formed ultramicrocellular structure is illustratedin the remaining examples.

All parts given in the examples are by weight unless otherwisespecified.

Example I A mixture of 400 parts of polyethylene terephthalate (relativeviscosity 45, dried 24 hours under vacuum at 80 C.), 325 parts methylenechloride, and 70 parts dichlorodifluoromethane inflatant is sealed intoa 1 liter pressure vessel which is rotated end-over-end to mix thecontents while being heated to 210 C. for minutes, and then for a periodof minutes at 191 C. At this point rotation is stopped, the pressurevessel connected to a source of nitrogen at 800 p.s.i.g., and thesolution extruded through a cylindrical orifice 15 mils diameter by 30mils long. Flash evaporation of the solvent generates a fully inflatedultramicrocellular filament as soon as the super-heated polymer solutionreaches the atmospheric pressure region.

A portion of the product (portion A) is piddled in a pile and allowed tostand in air at room temperature. A second portion (B) is collected inan air impermeable plastic bag filled with a dichlorodifluoromethaneatmosphere. Both portion A and portion B collapse initially as methylenechloride (the predominant gaseous component) diffuses rapidly out of thecells, leaving behind only the slower diffusing dichlorodifluoromethane.Portion A subsequently self-reinflates as air permeates into the cells.Portion B, however, remains in a partly collapsed condition, since thereis no osmotic driving force for the external dichlorodifluoromethane todiffuse into cells already containing an atmosphere ofdichlorodifluoromethane. After three days storage in collapsed form,portion B is removed from the bag and its density determined to be 0.068g./cc. compared to 0.022 g./cc. for self-reinfiated portion A. After oneadditional day, standing exposed to air, portion B also self-reinflatesto a density of 0.022 g./cc. The average cell diameter of the fullyinflated yarn is 23 microns, and the wall thickness is 0.12 micron. TheNc-s value for the collapsed portion B is about 45. The cell walls ofboth samples are found to exhibit uniform texture and uniplanarorientation.

Example II In three companion experiments, 40% solutions ofpolypropylene in fluorotrichloromethane are prepared in a pressurevessel at l45il C. The solutions are pressurized with nitrogen gas to570 p.s.i.g. just prior to extrusion through a 50-mil diameter by 50-millong cylindrical orifice. Since fluorotrichloromethane permeatespolypropylene cell walls only slightly faster than air, the cells of theproduct from spin A containing no impermeant inflatant additive fillwith air about as fast as the fluorotrichloromethane activating liquidescapes. The product therefore is not fully expanded, i.e. has a densityof about 0.017 g./cc. In spins B and C 10% of the fluorotrichloromethanein the pressure vessel solution is replaced with impermeant inflatant, amixture of isomers of perfluorodimethylcyclobutane in spin B and CF CFCF OCFHCF in spin C. The respective ultramicrocellular products of spinsB and C thus become fully inflated to densities of 0.010 and 0.012g./cc. within 3-4 minutes of being extrudedpThe density of sample Bremains substantially unchanged for at least the next 40 hours. Duringthis same period the density of sample C passes through a maximum ofabout 0.019 g./cc., but returns to a stable density of about 0.013g./cc. within 40 hours.

Example III When a fast permeating activating liquid is employed,initial outward diffusion of vapor is very rapid and appreciable degreesof collapse are achieved before reinflation with air occurs.

Polypropylene (containing 1% silica aerogel) and methylene chloride aresupplied at a 50/50 weight ratio to a 2" screw extruder where they areblended and heated to form a solution at C. This solution is suppliedunder 1200 p.s.i. pressure to an orifice 30 mils diameter by mils longand extruded into a region at atmospheric pressure and ambienttemperature. Flash evaporation of the solvent generates a cellularfilament having polyhedral cells and uniplanar orientation. Rapidoutward diffusion of the methylene chloride vapor leaves the filamentspartially collapsed at a density of 0.042 g./ cc. The collapse occurswithin a few seconds of extrusion, and leaves the product with wrinkledand buckled cell walls.

In a companion experiment, symmetrical dichlorotetrafluoroethaneinflatant is added to the methylene chloride at a 20/100 weight ratioand the solution temperature adjusted to 140 C. The orifice andextrusion pressure remain unchanged. The ultramicrocellular filamentgenerated by the intial flashing of the liquid into vapor again rapidlycollapses as the methylene chloride vapor diffuses out. However,appreciable quantities of the dichlorotetrafluoroethane inflatant remainin the cells and hence facilitate osmotic diffusion of air into thecells, i.e. the sample self-reinflates spontaneously in air at roomtemperature. By the time the first density determination is performed(15 minutes after extrusion) the product has already re-expanded to adensity 0.022. g./cc.; and 20 hours after extrustion the fully expandeddensity of 0010:.001 g./cc. has been reached, and remains constant forat least the next 200 hours.

At any time prior to 15 minutes after extrusion, the transient partiallycollapsed dichlorotetrafluoroethanecontaining sample may be trapped bymechanically confining the sample or by removing it from contact withthe air, e.g., by placing it in an air impermeable container, or in anair-free impermeant inflatant atmosphere. At this point the sample has aNc-s value in excess of 30- and contains sufficientdichlorotetrafluoroethane in its cells so that self-reinflation willoccur on subsequent unrestricted exposure to air.

Example IV A pressure vessel is charged with 1,000 grams polypropyleneof melt flow rate 4 at 230 C., 750 ml. of methylene chloride and 5 gramssilica aerogel. Chlorotrifluoromethane is equilibrated with the mixtureat a pressure of 40 p.s.i.g. The pressure vessel is sealed, heated to150 C., and rotated end-over-end to mix the contents. At thistemperature the contents form a solution with a vapor pressure of 300p.s.i.g. The pressure vessel is positioned vertically and connected to asource of nitrogen gas at 350 p.s.i.g. just prior to extrusion of thecontents through a -rnil diameter orifice. Flash evaporation of thesuperheated liquid solvent generates a low density cellular strandhaving closed polyhedral cells with walls less than 2 microns thickexhibiting uniform texture and uniplanar orientation of the polymer. Aportion of the product is collected in a plastic bag containing achlorotrifiuoromethane atmosphere (portion A) and another portion (B) iscollected in air. Both initially fully expanded portions collapse withina few seconds as methylene chloride vapor rapidly diffuses out of thecells. Subsequently, portion B self-reinflates as air permeates into thecells. Portion A on the other hand remains collapsed. Two weeks later,the still collapsed portion A is exposed to air, and within 24 hoursself-reinflates by a 3.8 fold volume expansion. The value of Nc-s forcollapsed portion A during its storage period in the bag is about 55.The self-inflation of both portions A and B occurs as air, driven by theosmotic pressure gradient existing across the cell walls permeates intothe cells faster than the slower diffusing chlorotrifluoromethaneinflatant permeates out.

Example V A rapid permeating activating liquid is prepared from a 25/75volume mixture of hexane/pentane. To 90 parts (by volume) of thisactivating liquid is added 10 parts of a mixture of isomers ofperfluorodimethylcyclobutanes, used as an impermeant inflatant. A weightpercent solution of polypropylene (melt flow rate 0.4 at 230 C.,containing 1% by weight of silica aerogel) is prepared in the activatingliquid/inflatant mixture in a pressure vessel at 141 C., and extruded at500 p.s.i.g. through a 50-mil diameter by 50-mil long cylindricalorifice into a region at atmospheric pressure and room temperature.Flash evaporation of activating liquid generates an inflatedultramicrocellular filament structure which rapidly collapses as theactivating liquid condenses and/ or diffuses out of the cells. Theresidual impermeant inflatant content in the cells creates an osmoticdriving force for postinflation by air. Three minutes after extrusion,the first density determination indicates the sample has alreadyinfiated to a density of 0.029 g./cc. After 6 hours standing in air, thesample has inflated to a density of 0.016 g./cc., at 22 hours thedensity is 0.012 g./ cc. and after 118 hours, the sample is fullyinflated (turgid) and has reached a stable density of 0.010 g./cc. and adiameter of about 10 millimeters.

For comparison, a similar experiment is performed omitting theimpermeant infiatant. Thus, a 50% solution of polymer is prepared at 142C. in a 25/ 75 (volume) mixture of hexane/pentane and extruded to yielda col lapsed cellular filament at a density of 0.04 g./ cc. having notendency to reinflate in air.

Example VI A polypropylene ultramicrocellular sheet is extruded at atemperature of 145 C. through a 3" annular die with a 10 mil gap from apressure vessel containing solution of polymer in a mixed solventconsisting of parts methylene chloride activating liquid and 10 partssymmetrical dichlorotetrafluoroethane inflatant. The sheet is wound upin a solvent vapor atmosphere at a temperature of 40 C, The roll ofinflated sheet is doffed, removed from the spinning cell and rewound inair at an ambient temperature of about 25 C. Diffusion of methylenechloride from the cells (as Well as condensation of residual vapor) isvery rapid and leads to partial collapse of the cells, which however,retain a substantial portion of the slower diffusingdichlorotetrafluoroethane. Before reinflation due to diffusion of airinto the cells can occur, the re-wound package is sealed into apolyethylene terephthalate film bag containing an atmosphere ofdichlorotetrafluoroethane. Whenever reinflation is desired, the bag isopened, the roll unwound, and the product exposed to air.

Example VII An inflatant/activating liquid mixture is prepared bybubbling gaseous SP (sublimes at 64 C.) through fluorotrichloromethanewhich is cooled by a Dry Ice/ acetone bath. The resultant slurry ismixed with polypropylene (melt flow rate 0.4 at 230 C., containing 2% byweight of silica aerogel) to give a 40% solution of the polymer. Themixture is heated in a pressure vessel to a temperature of 142 C., andextruded at 500 p.s.i.g. through a 50-mil diameter by 50mil longcylindrical orifice into a region at atmospheric pressure and roomtemperature. The inflated ultramicrocellular filament initially formedpartially collapses within a few moments, the density of the filamentone-half hour after extrusion being 0.021 g./cc. Upon continued exposureto air at room temperature for 3 days, the sample inflates to a densityof 0.015 g./cc.

Example VIII This example illustrates the utilization of a polymerhaving a Tg in excess of 40 C. and the production of a collapsedultramicrocellular structure which does not self-inflate in air untilheated.

A mixture of 400 grams polyethylene terephthalate (Tg 69 C., sampledried at 110 C. in a vacuum oven), 325 grams methylene chloride(activating liquid) and 70 grams 1,1,2-trichloro-1,2,2-trifluoroethane(impermeant inflatant) is heated in a 1 liter pressure vessel to atemperature of 220 C. and a uniform solution is formed by rotating thepressure vessel end-over-end. The autogenous pressure of 720 p.s.i.g. isincreased with nitrogen gas to 800 p.s.i.g. just prior to extrusionthrough an orifice 0.005 inch diameter by 0.025 inch long into a regionat atmospheric pressure. Flash evaporation of the activating liquidgives an ultramicrocellular filament. The filament, generated at therate of 1900 y.p.m., is led to a windup'operated at 2020' y.p.m., andwound on a bobbin under slight tension. Most of the methylene chloridevapor permeates out of the filament between the extrusion orifice andthe windup, and the remainder condenses at room temperature, so that asubstantially collaped closed cell filament is wound-up directly. Thetension of the yarn on the package prevents air from permeating thetrifluorotrichloroethane-containing cells and reinflating the structure.The sample is allowed to stabilize during two days storage at roomtemperature. The collapsed yarn is now metastable and may be removedfrom the bobbin (i.e., tension removed) without reinflation occurring.The density of the yarn removed from the outside of the package is 0.12g./cc., while the still more tightly confined yarn on the inside of thepackage has an even high density of 0.23 g./cc. Both of these metastablesamples reinflate to a density of 0.046 g./cc. on being heated in a C.air oven. A portion of the collapsed yarn removed from the bobbin andstored under no mechanical restraint at room temperature for one Weekretains its density of 0.23 g./cc. but inflates to a density of 0.046g./cc., as before, on being heated only 5 minutes at 100 C. The residualtrifluoro- N is estimated from =2.3 10 0111. per cell 0.23 8 Nc 1.3 10 x6.5 10 cell/cc.

Therefore, Nar :73 for this sample.

Example IX Polypropylene ultramicrocellular filaments spun fromsolutions without impermeant inflatants are obtained in substantiallycollapsed form. Thus, 50% solutions (containing 1% silica aerogel basedon the polymer weight) in hexane at 136 C. and in 2,2-dimethylbutane at145 C. are extruded through a 50 mil spinneret to yield partiallycollapsed ultramicrocellular filaments of density 0.081 and 0.042 g./cc.respectively. These samples are subsequently placed in a 3-literpressure vessel charged with 300 ml. of methylene chloride and 300 ml.of perfluorocyclobutane, the vessel sealed and heated to a temperature(approximately 55-60 C.) which yields an internal pressure ofapproximately 120 p.s.i.g. This treatment gives rise to a temporaryplasticizing effect upon the cell walls such that perfluorocyclobutaneis caused to penetrate the closed partially collapsed cells of thesamples. When the pressure vessel is subsequently cooled and the sampleremoved, the methylene chloride rapidly diifuses away, leaving theperfluorocyclobutane locked in the cells. Thus, an osmotic pressuregradient exists which drives air into the samples to fully reinflateboth of them to a density of 0.007 g./cc. The inflatant half-life forsuch samples e.g., the time required for half of the originalperfluorocyclobutane to diffuse out and be lost to the atmosphere is onthe order of 40 days or more.

Example X A plastic bag containing fully inflated polypropyleneultramicrocellular fibers of density 0.017 g./cc. is hung in a chamber.Symmetrical dichlorotetrafluoroethane is introduced and refluxed in thechamber. As the air in the chamber is displaced by thedichlorotetrafluoroethane vapor, the fibers collapse by loss of air,since air permeates out of the cells faster thandichlorotetrafluoroethane permeates in. Within 1 /2 hours the density ofthe fibers has increased to 0.037 g./cc. The excess vapor in the bag ismechanically expelled, and the bag sealed to prevent air from contactingand reinflating the collapsed fibers when the bag is removed from thechamber. At this (point the bag occupies a relatively small volume. Whenthe fibers, which now have a quantity of dichlorotetrafluoroethane intheir collapsed cells, are subsequently =re-exposed to air, they beginto self-reinflate and after standing only overnight, have alreadyexpanded to a density of 0.023 g./cc. This technique may be used toprovide expanded-in-plate insulation for refrigerators and the like, aswell as effect savings in shipping and storage.

Example XI Polyethylene terephthalate ultramicrocellular filaments areprepared in a continuous process by feeding polymer chips of RV 50(dried 36 hours at 165 C.) to an extruder at a rate of 196 parts/minute.The chips are advanced and compressed by a rotating screw, heated tomelt the polymer which is then mixed with methylene chloride suppliedfrom an auxiliary pump at 105 parts/minute to form a solution at 212 C.which is extruded under a pressure of 800 p.s.i.g. through a cylindricalorifice 12 mils in diameter by mils long.

These polyethylene terephthalate filaments, like the polypropylenefilaments of Example IX, contain no spunin impermeant inflatant.Accordingly, skeins of the filaments are immersed for 20 minutes in asolution of chloropentafluoroethane in methylene chloride refluxing at 5C. under 1 atmosphere pressure. The skeins are then transferred directlyto a 70 C. water bath for 60 seconds to strip off the methylene chlorideplasticizer (trapping the chloropentafluoroethane impermeant inflatantinside the cells) and then heated in air at C. for 30 minutes toaccelerate full reinflation by air diflusing osmotically into the cells.In an analogous procedure prefluorocyclobutane (another impermeantinflatant) is introduced into several skeins of the as-spun filaments byemploying a perfluorocyclobutane/methylene chloride bath refluxing at 6C. and a 30 minute immersion time. In each case the fully inflatedfilaments contain approximately .3 atmosphere partial pressure of theimpermeant inflatants inside the cells plus approximately 1 atmospherepartial pressure of air.

These polyethylene terephthalate ultramicrocellular filaments possessexcellent cushioning properties which are extraordinarily durable.Recovery from momentary (cyclical) loads is excellent. Furthermore, if adead load is applied long enough for some of the air to have difiusedout of the cells, on removal of the load the sample will tend toself-reinflate to its original condition as external air is osmoticallyreintroduced into the cellsso long as the impermeant inflatant has beenretained within the cells. A particularly rigorous arbitrary test hasbeen developed to evaluate quantitatively this cushioning performancefeature. A parallel array of contiguous inflated filaments is subjectedto a dead load of 200 Psi. applied to the face of the array for a periodof one week. The initial impermeant inflatant content. Table I recordsthe sion as air diffuses out of the cells. After one week the load isremoved, and the initial rapid recovery is followed by a slow additionalrecovery for one week as air re-enters the cells. The percent recoveryof the initial thickness of the array is measured, as well as thepercent loss of the initial impermeant inflatant content. Table Irecords the test results for polyethylene terephthalateultramicrocellular filaments containing a variety of impermeantinflatants.

Perfluorocyclobutane and chloropentafluoroethane are clearly preferredas having a good (high) recovery and desirable (low) loss of inflatantfrom the sample. It is, of course, true that for those cushioning orother applications where such extreme loads are not encountered, many ofthe other impermeant inflatants in the table will perform very well, andmay in fact be preferred for other reasons.

I claim:

1. In a method for producing an ultramicrocellular structure composed ofa high molecular weight synthetic crystalline polymer capable of forminga film and having a major proportion of closed polyhedral cells definedby air permeable walls having a thickness of less than 2 microns withessentially all the polymer constituting cell walls and exhibitinguniform texture and uniplanar orientation, said method comprising flashextruding a solution of said polymer in an activating liquid, saidsolution being maintained at superatmospherie pressure and at atemperature above the boiling point of said activating liquid and beingdischarged from an orifice into a region of lower pressure andtemperature to precipitate said polymer, the improvement for obtaining aproduct which is self-inflatable in air comprising providing in saidsolution an inflatant whose permeability coefficient for diffusionthrough said walls is less than that of air, said inflatant beingcapable of generating a vapor pressure of at least 30 mm. Hg at atemperature below the softening point of the polymer and being selectedfrom the group consisting of sulfur hexafluoride and saturated aliphaticand cycloaliphatic compounds having at least one fluorine to carboncovalent bond and wherein the number of fluorine atoms exceeds thenumber of carbon atoms.

2. The method of claim 1 wherein said polymer is polypropylene; saidactivating liquid is selected from the group consisting of methylenechloride, alkanes of 4 to 7 carbons and mixtures of said alk-anes, andsaid inflatant is selected from the group consisting of perhaloalkanesand perhalocycloalkanes in which at least 50% of the halogens arefluorine.

3. The method of claim 1 wherein said solution contains about 1 to 20%of said inflatant.

4. In a method for producing an ultramicrocellular structure composed ofa high molecular weight synthetic crystalline polymer capable of forminga film and having a major proportion of closed polyhedral cells definedby air permeable walls having a thickness of less than 2 microns withessentially all the polymer constituting cell walls and exhibitinguniform texture and uniplanar orientation, said method comprising flashextruding a solution of said polymer in an activating liquid, saidsolution being maintained at superatmospheric pressure and at atemperature above the boiling point of said activating liquid and beingdischarged from an orifice into a region of lower pressure andtemperature to precipitate said polymer, the improvement for obtaining acollapsed product which is self-inflatable in air to less than one-halfits collapsed density without substantial stretching of the cell wallscomprising (a) providing in said solution an inflatant whosepermeability coefficient for diffusion through said walls is less thanthat of air, said inflatant being capable of generating a vapor pressureof at least 30 mm. Hg at a temperature below the softening point of saidpolymer and being selected from the group consisting of sulfurhexafluoride and saturated aliphatic and cycloaliphatic compounds havingat least one fluorine to carbon covalent bond and wherein the number offluorine atoms exceeds the number of carbon atoms and (b) followingdischarge from said orifice and while the precipitated structure is in atransient collapsed condition such that its Nc product is greater thanabout 30 Ne being the number of cells per cc. in the collapsed structureand s being the average cell surface area in square centimeters,restraining self-inflation by preventing osmotic diflusion of air intothe cells of said structure.

5. The method of claim 4 wherein said self-inflation is restrained byenclosing said precipitated structure in a gas impervious barrier means.

6. The method of claim 5 wherein said gas impervious barrier meanscomprises a sealed container having disposed therein an impermeant gaswhich envelops said preeipitated structure.

7. The method of claim 6 wherein said sealed container is a plastic bag.

8. The method of claim 6 wherein said impermeant gas is the same as saidinflatant.

9. The method of claim 4 wherein said polymeric solution contains about1 to 20% of said inflatant.

References Cited UNITED STATES PATENTS 3,102,865 9/1963 Sneary et al26453X 3,227,784 1/1966 Blades et al 264-53 3,344,221 9/1967 Moody et al264-53X 3,384,531 5/1968 Parrish 264-53X PHILIP E. ANDERSON, PrimaryExaminer US. Cl. X.R. 264-53, 176

gg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 9Dated June 8, 1971 Inventor(s) Robert Guy Parrish It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 20 Claim 14, line 11, "No should read Nc s /2 Signed and sealedthis 19th day of October 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Patents

